During a sensing application or in connection with the tracking of moving objects by a digital camera, it is customary to use a mode for reading the sensor thereof called “subresolution” mode.
In the context of the present invention, movement means rapid and wide variations in luminosity of a portion of the image, induced, inter alia, by an actually moving object of the real world, or by the variation in luminosity of a light source, like a flashing light, for example.
This type of “subresolution” readout consists in subdividing the two-dimensional matrix of light sensitive elements of the camera or pixels, into blocks (referred to as “macropixels”) and in reading all the pixels of the macropixels (referred to as high resolution readout mode) of a predefined zone of the matrix comprising the moving object. The other macropixels are then read in low resolution mode, consisting for example in reading only one pixel per macropixel.
This composite readout mode, that is a high resolution read out of the macropixels of interest, and the low resolution readout of the other macropixels, thus has the effect of substantially reducing the quantity of data to be processed. This therefore leads to lower power consumption and a lesser need for computation resources.
Such a readout mode is described for example in document US-A-2004/0095492. In this document, high resolution is applied to a target object. For example, a detection mode is used to find a light spot in the image and to determine the elements thereof to be read in high or low resolution. The update of the high resolution zone is then determined by analyzing the movement on the basis of the pixels read in high resolution mode.
The detection or tracking of moving objects has a particular application in the field of infrared viewing using a digital camera having a bolometric sensor. In fact, the use of such a camera, which is sensitive to infrared, allows surveillance of areas, such as parking lots for example, independently of the time of day, unlike visible detectors which require sufficiently lighted scenes.
Bolometric sensors are devices arranged in matrix form, and capable of operating at ambient temperature, that is not requiring any cooling to very low temperatures.
These uncooled sensors conventionally use the variation of a physical quantity of an appropriate material, as a function of temperature, in the neighborhood of 300K. In the case of bolometric sensors, this physical quantity is the electrical resistivity.
Sensors for infrared imagery are conventionally made in the form of a matrix of elementary sensors, or bolometers, said matrix being suspended above a substrate, generally made from silicon, via support arms.
Means for sequential addressing of the elementary sensors and of the means of electrical excitation and for preprocessing the electrical signal generated by these elementary sensors, are usually arranged in the substrate. This substrate and the integrated means are commonly referred to as the “readout circuit”.
In fact, bolometers are observed to display a dispersion of their response, a dispersion which is also accentuated or varies over time.
Thus, the image of a uniform scene by a bolometric sensor is generally not a uniform image. This situation is referred to as “offset” dispersion. Similarly, a dispersion is observed in the gains of the bolometers, since the bolometric sensor image of a uniform variation of a uniform scene is not uniform.
There are many reasons for such a dispersion, but mention can be made in particular of the illumination of the sensor by a high flux, as for example during the observation of an intense radiation source (sun, floodlight, etc.), which causes a phenomenon of durable afterglow, which is detrimental to the quality of the thermal images delivered by the sensor.
Also worth mentioning is the fact that, among a given batch of imaging bolometers, a natural dispersion exists in the electrical resistances, which is more or less pronounced according to the method and the materials used for the fabrication thereof. Hence, from its initial use, a sensor comprises a spatial dispersion of its detection characteristics.
In fact, a 1% deviation of the electrical resistance of an imaging bolometer from a reference resistance causes an error of about 10 to 20% in the temperature of the scene estimated through it.
Since, furthermore, the value of the useful signal only usually accounts for about 10% of the total dynamic range of the bolometric sensor, it can be easily understood that the dispersion of the detection characteristics of the bolometric sensor seriously jeopardizes detection quality.
In general, it is observed that an uncorrected image of a bolometric sensor is difficult to use visually.